Different vendors of accessories for mobile telephones and other mobile devices have built-in unique resistors values that identify the different accessories. When attached, the mobile device detects the value of the resistor that identifies the specific accessory. The operation of that accessory may then be executed without further actions by a human user.
Limitations exist when designing systems to measure these resistors. More specifically, the dynamic range of the resistors involved place limitations on minimum currents and/or voltages involved.